A new method for quantifying Puccinia triticina airborne inoculum in wheat fields

Wheat leaf rust caused by Puccinia triticina is regularly observed in Belgian fields. This fungus produces wind-dispersed urediospores that infect host plants many kilometers from their source plant, but the origin of the primary inoculum and the factors involved in disease occurrence and severity are not well known. In order to better understand the epidemiology of the disease, the dispersion of P. triticina airborne inoculum and the relationship with disease incidence in fields were studied. A network of Burkard 7-day spore-recording traps was set up in wheat fields in the Walloon region, in Belgium, to monitor the airborne inoculum. Total DNA from each fragment of spore trap tape, corresponding to 1 day of sampling, was extracted and the quantity of P. triticina on the tape fragments was assessed using a specific real-time polymerase chain reaction (PCR) assay. The disease incidence was recorded weekly in experimental fields from March 2012 onwards. Initial analysis revealed that the real-time PCR assay was highly specific and repeatable. The detection threshold was determined at 7 spores per m3. This method (spore traps coupled with real-time PCR) allows airborne inoculum to be detected before the appearance of symptoms in fields and could be used in a model for predicting wheat leaf rust epidemics

Prevalence of the species involved in Fusarium head blight (FHB) in winter wheat in Wallonia, Belgium from 2010 to 2012

Fusarium head blight (FHB) is a common plant disease occurring frequently in winter wheat in Belgium and associated with yield losses and mycotoxin contamination. The pathosystem involves a complex of species in the Fusarium and Microdochium genera. In order to evaluate the occurrence and distribution of the causal species in Wallonia, ears of winter wheat were collected in five experimental fields in 2010, 2011 and 2012. Grains from three positions along the rachis (top, middle and bottom) of each ear were investigated for the presence of FHB pathogens, using a plate isolation technique and monoconidial cultures. The species were identified using morphological and molecular (elongation factor 1-) characterization. Both infection prevalence and species occurrence differed from one location to another and from year to year, with one or more species being predominant each year. Some species were present one year, but absent in the following year. For example, in 2011, F. culmorum was the most predominant species, being present in every location, but it was rarely isolated in 2012 and it was absent in 2010. In each year F. graminearum, F. poae and F. avenaceum were isolated; F. poae was the prevalent species in 2010, and F. graminearum and F. avenaceum were the prevalent species in 2012. A rare and highly toxigenic species, F. langsethiae, was isolated in 2010 and 2012. In each year the proportions of FHB infection differed significantly (p < 0.01) between the three positions along the rachis, with an increasing intensity from top to bottom. The infections caused by F. graminearum, however, did not quite follow this gradient, with a slightly greater amount of infection found at the top than at the bottom. This discrepancy could result from the ability of this species to produce airborne ascospore inoculum. Our results highlight the great variation in the composition of the FHB complex, suggesting that the epidemiological characteristics differ from one species to another. Although the plate isolation technique is time consuming, it allowed a pathogen culture collection to be created that could be used for further characterization of the species and could improve the understanding of the epidemiology as well as disease/mycotoxin prediction

Four years of monitoring Mycosphaerella graminicola airborne inoculum and its relationship with disease symptoms in the field

Mycosphaerella graminicola is the causal agent of Septoria tritici blotch (STB) in winter wheat. This disease is called a ‘gradient disease’ because it generally progresses from the lower to upper leaf stages. Airborne ascospores originating in stubble or trash are now considered to be the most important source of primary infection in autumn and winter, whereas the subsequent development of STB occurs through the infection of the upper leaves by splash-dispersed pycnidiospores produced on the lower leaves. Recent reports and observations, however, question the importance of ascospores in the progress of STB epidemics after stem elongation in wheat plants. A method based on real-time polymerase chain reaction (PCR) assay and using Burkard spore traps was developed to quantify M. graminicola airborne inoculum. The method was tested for its reliability and applied in a spore-trap network over a 4-year period in order to investigate the spatio-temporal distribution of airborne inoculum in Belgium. A seasonal distribution was observed, with the highest quantities trapped between June and mid-August and with clusters detected from September to April. Depending on the year or site, significant quantities of airborne inoculum were measured during the cereal stem elongation and heading stages, suggesting that ascospores contribute to the infection of upper leaves later in the season. When many peaks of airborne inoculum were detected during this period, the disease gradient due to the vertical progression of splash-dispersed pycnidiospores was not always well marked. The STB incidence observed on an upper leaf layer was sometimes greater than the incidence on a lower leaf layer, suggesting that the airborne inoculum accelerates the progression of the disease on the upper leaves of wheat plant. The exploration of the relationship between disease evolution, airborne inoculum detection and meteorological conditions would provide valuable information for integrated crop protection